This invention relates to electrical apparatus, particularly to apparatus adapted for use as a clock extraction system.
When making measurements on data streams, e.g., bit error rate, it is necessary to extract the clock signal from the data in order to provide timing for the decoding circuits. Systems used for this purpose need to allow for both jitter on the received signal and also to ignore any noise which may be present. There is of course in general no regular clock pulse received with the data signal and the clock must be extracted by detecting edges on the received data.
Methods currently available for clock extraction use either tuned circuits, injection locked oscillators or resettable counters or oscillators. All of these systems are to some extent subject to perturbation by noise. Furthermore, tuned circuits and injection locked oscillators are designed for use with data rates at one particular frequency and are unsuitable for multifrequency systems. The present invention proposes a method of clock extraction, useful over a range of frequencies, in which the effects of interfering signals are reduced to a minimum and noise spikes are masked so that their interference with the data stream is substantially reduced.